One of the most important products produced in a steel mill is rod in sizes varying from 3/16" to 1" and varying in metallurgical characteristics that depend on the use for which the rod is intended. Small sizes are often drawn into wire, while the larger sizes are often used in screw machines for producing bolts and the like. It is desirable, therefore, that a given rod mill not only be able to produce good quality rod over the entire range of commercial sizes, but also over the entire range of types of steel, including low carbon, high carbon and alloy steels.
Besides the need for obtaining this versatility in a rod mill, the improvements made in the roll stands and the guide equipment have allowed higher and higher rolling speeds (in excess of 350 feet per second), which, of course, has resulted in problems in the downstream handling of the hot rod. Generally speaking, it is easily possible to quickly cool the rod in water boxes sufficiently to permit it to be coiled in a pouring or laying reel, but the major problems that have developed have to do with cooling subsequent to coiling under controlled conditions to obtain a variety of desired metallurgical characteristics. The development of the Stelmor process by the Morgan Construction Company of Worcester, Mass., has gone far to improve this desired control of the cooling and this process would be adequate if the rod mill were used for only a limited number of types of rod. Unfortunately, present day steel mills are required (for reasons of economy) to not only produce rods over a wide range of sizes and steel types, but also to be changed from one type to another very rapidly. Also, metallurgical and mechanical uniformity of the rod product have become important components in judging the acceptability of rod quality.
Now, in a typical rod mill the cooling takes place on an elongated conveyor on which the coil is dropped from a laying reel as the conveyor moves, so that the rod lies on the conveyor in overlapping offset rings or coils. Cooling air is provided to the underside of the conveyor and flows upwardly through the staggered rings of rod. By varying the air flow at various positions along the length of the conveyor, it is possible to control the rate of cooling to a certain extent.
Unfortunately, this general construction of air-cooled conveyor system does not render the rod mill capable of producing the wide range of sizes and metallurgical types that is most desirable in order to use the rod mill most efficiently. This is largely due to the structure of the conveyors that have been used in the prior art systems. To begin with, these conveyors have consisted of the chain type and the roller type.
The chain type cooling conveyor has consisted of sprocket-driven chains which have upwardly-extending dogs that engage the rings of rod and drag them along over support rails. The roller type has consisted of spaced, parallel driven rollers that support the series of rings and move them along the conveyor.
Chain conveyors have more efficiency than the roller type, since they generally have a gap between the rod stock and the air nozzle opening. This gap allows air to flow both over and under the conveyed rod stock. The number of nozzles, their angles, and the nozzle throat and exit configurations also contribute to the cooling rate. The chain-type conveyor allows more freedom longitudinally to place an optimum number, angle, and frequency of nozzles. Unfortunately, the chain-type conveyor has several major deficiencies. The chain dogs and skid rails tend to mark the rod stock, the dogs tend to group the rings in irregular patterns and the ring overlaps are fixed, i.e., do not move during transit. When the ring overlap points are fixed, the intersecting mass areas are difficult to cool. Cool chains also act as a heat sink and tend to cool stock more rapidly when it contacts the chain.
The present practice is to make the nozzle decks from castings. This practice limits the size and number of nozzles in each deck section. Roller conveyors, on the other hand, allow the overlapping rod rings to shift, since a speed change or a ring drop can be used to produce a ring overlap shift, thus optimizing the rod cooling pattern. The roller conveyor can maintain a uniform ring pattern, even when there is a ring shift. Such even ring patterns keep the rings open for free flow of air which enhances uniform rod cooling. An unmarked rod is a major criterion for prime quality rod and the roller-type conveyor is least likely to mark the rod, particularly when the rod is in the red-hot range. The roller-type conveyor allows the stock to be easily centered by "persuader" rolls, but persuader roll adjustment is limited by roller spacing. Present roller conveyor designs serve to limit cooling efficiency, because the roller acts as a dam and does not permit air to pass freely and effectively along the bottom side of the stock. Air can pass vertically through the stock or pass over the top. Some improvement in cooling between the rollers is done by causing turbulence in the air flow, for instance, by blowing the air under the roll and allowing it to eddy up and around the roller. Even with large amounts of air, achievable cooling rates are lower than over and under cooling done generally on chain-type conveyors. Too large a nozzle pressure with a shallow nozzle angle can cause the stock to lose traction and to slide in an uncontrolled manner on a roller type conveyor.
The concept of placing a cooling source over the conveyor is unacceptable, since such a system generally obstructs the visibility and prevents easy accessibility to damaged stock. Therefore, cooling air on all types of conveyors are restricted to doing so from the underside.
Where chain-type conveyors allow the rod rings to drag on surfaces (instead of skids), the resulting cooling efficiency is as low (if not lower) than with the roller-type conveyor.
During slow cooling processes, as compared with rapid cooling, radiant heat from the hot rod stock is contained within an enclosed conveyor, the conveyor top and side walls being usually insulated. The bottom of the conveyor usually contains the stock heat by blocking any direct radiation from escaping into the air chamber below. The rollers in such a slow-cool situation are subjected to high temperatures. Therefore, the rollers are usually oversized to prevent them from creeping or sagging. The ends of the rollers must either have water-cooled bearings or have finned ends to prevent overheated outboard bearings.
With the chain-type conveyor, the chains have limited life under slow-cool conditions, and chain length expands considerably, causing take-up and sprocket jamming problems. So slow-cool conveyors have at present been limited to the roller type. In such cases, the air nozzles are sometimes designed to limit radiation loss and are either placed under the rollers or have a nozzle-closing mechanism Nozzle patterns are usually fixed and generally supply more air to the edge of the ring pack where the stock is densest. One system has been known to use nozzle inserts or blocks to develop an operating pattern. Other systems have continuous nozzles running from side-to-side and rely on divided chambers under the conveyor deck to provide a greater air flow to the edge of the ring pack.
It can be seen, then, that a chain-type conveyor, when used in the "slow-cooling" processes, has a short life, while a roller-type conveyor under such conditions requires extraordinary complications in structure. At the same time, neither type of conveyor operates effectively under "rapid cooling" processes. It should also be pointed out that any attempt to extend the range of cooling capability by increasing the fan capacity is not an acceptable alternative, because of the cost of larger motors and the noise accompanying the use of larger fans. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a rod cooling system in which controlled cooling can take place over a wide range of sizes and metallurgical requirements.
Another object of this invention is the provision of a system for conveyor-type rod cooling which is efficient during both slow cooling and rapid cooling.
A further object of the present invention is the provision of a rod cooling system for a rolling mill, which system is simple and rugged in construction, which can be easily and inexpensively manufactured from readily-obtainable materials, and which is capable of a long life of useful service with a minimum of maintenance.
A still further object of the invention is the provision of an air-cooling system for a rolling mill which operates effectively with a minimum of fan capacity.
It is a further object of the invention to provide a cooling means for overlapping rod coils, wherein the contact points between coils are changed frequently and wherein cooling takes place equally at all parts of the coil.
Another object of the invention is the provision allowing for a flexible choice of nozzles along the length of the conveyor which optimizes air flow control.
Another object of the invention is the provision of a rod cooling conveyor whose active elements weigh less than the equivalent roller type element.
Another object of the invention is the provision of a rod cooling system that can be easily used in converting an existing rod cooling apparatus.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.